ClangImporter: run Clang with a proper executable path

Clang deduces its installation directory from the `argv[0]` parameter (see clang/lib/Frontend/CreateInvocationFromCommandLine.cpp), and the default include search paths are computed based on the installation directory.

This change allows compiling Swift code that imports the C++ stdlib without having to manually specify the include search path of `std` headers.

include/swift/Basic/LangOptions.h

@@ -597,6 +597,10 @@ namespace swift {
   /// Options for controlling the behavior of the Clang importer.
   class ClangImporterOptions final {
   public:
+    /// The path to the Clang compiler executable.
+    /// Used to detect the default include paths.
+    std::string clangPath = "clang";
+
     /// The module cache path which the Clang importer should use.
     std::string ModuleCachePath;